Conventionally, an extrusion screw for a melt extruder comprises an elongate shaft (root) with a helical thread (flight) extending therealong, and comprises a feed section, a melting section (also called a transition section), a metering section and a mixing section. The melting or transition section is disposed between the feed section and the metering section, and the metering section is disposed between the melting or transition section and the mixing section. The extrusion screw is driven to rotate within a smooth-bored barrel.
Plastic, typically as pellets, is fed into the barrel at the feed section and carried into the melt or transition section by the movement of the helical threads, where the plastic melts and begins to mix, and then into the metering section, which advances the molten plastic at a desired rate into the mixing section, where it is thoroughly mixed and can be extruded at the appropriate rate through an extrusion aperture.
The lead or pitch of the thread may vary along the length of the screw to impart the described characteristics; for example, the lead and pitch may be larger in the feed section than in the melting section and larger in the melting section than in the mixing section.
It is known in the prior art to provide an extrusion screw in which the shaft of the mixing section has, instead of threading, a longitudinally extending, longitudinally spaced series of annular sets of circumferentially spaced teeth, with the teeth being of generally rectangular parallepipedic shape and extending radially outwardly from the shaft.